Apparatus with ambient magnetic field correction

ABSTRACT

This atomic clock comprises means for applying two mutually perpendicular oscillating magnetic fields ( 9, 10 ), governed by a control device ( 5 ) that makes them apply a static or nearly static magnetic field for compensating the ambient magnetic field in order to cancel sub-level energy variations of the matter, which disrupt the frequency of the returned photons and the reference provided by the clock. Traditional magnetic shielding may be omitted. Said device can also operate as a magnetometer.

This invention relates to an apparatus comprising a correction of theambient magnetic field; it is added that this apparatus is used in anatomic clock for measuring time, or in a magnetometer, by a simplemodification of the operating instructions.

Atomic clocks comprise a gaseous medium, often alkaline, a device forexciting the atoms of this gas such as a laser, capable of making themjump to higher energy states, and a means for measuring a frequentialsignal emitted by the atoms on returning to the normal energy level,using the photons coming from the laser.

The frequency of the signal of the photons returned by the gas isdefined by the formula v=ΔE/h, where v is the frequency, ΔE thedifference between the energy levels and h Planck's constant, equal to6.62×10⁻³⁴ J/s.

It is known that this frequency is very stable and that it can thusserve as time reference unit. This is however no longer true when theZeeman structure of the material is considered: the energy levels thenappear as composed of sub-levels corresponding to slightly differentstates, which are distinguished by their magnetic quantum number m, 0for a reference state of the energy level and −1, −2, etc. or +1, +2,etc. for the others. This is illustrated by FIG. 1 in the case of theelement ⁸⁷Rb, in which has been shown the breakdown of the first twoenergy levels (of angular moments F=1 and F=2).

The energy levels are sensitive to the ambient magnetic field. Thissensitivity is low (of the second order) for the sub-level at themagnetic number equal to 0, but much greater (of the first order) forthe other sub-levels: the transitions made from or up to them producephotons, the frequency of which is variable and thus cannot serve asreference, and only the portion of the signal corresponding to thetransition between the two sub-levels of zero magnetic number isexploited for the measurement, which adversely affects its quality. Thereference frequency given by the clock is then the hyperfine transitionfrequency considered in the gas fo=E_(o)/h, where E_(o) is the energydifference between the sub-levels at m=0 of the two states (F=1 and F=2in the example of FIG. 1).

One thus resorts to a magnetic shield around the clock to reduceexterior perturbations and to the application of a constant magneticfield to properly separate the sub-levels, for want of guaranteeing azero magnetic field. Although the operation of the clock is made morestable, the sub-levels then being immobile and thus well defined, thedrawback of undergoing a dispersion of the frequencies and having tomake do with a weakened signal is not avoided.

With the invention, it is endeavoured to improve existing atomic clocksby making them work in zero magnetic field in order to concentrate thesub-levels at a same energy value and to obtain a signal comprising amuch sharper measurement peak. These considerations apply without changeto other apparatuses and in particular to magnetometers, to which theinvention thus also applies.

It consists in an apparatus that can serve as atomic clock ormagnetometer, comprising a cell filled with a gas, an exciter of the gasto make its atoms jump to a higher energy level, a detector to collect alight signal passing through the gas, characterised in that it comprisesmeans for applying magnetic fields, applying an essentially staticmagnetic field and two oscillating magnetic fields and directedperpendicular to each other, and means for controlling the means forapplying magnetic fields to regulate in direction and in intensity theessentially static magnetic field.

The invention will be described in a more complete manner with referenceto the following figures:

FIG. 1, already described, illustrates an energy level diagram of anelement of a material used in an atomic clock;

FIG. 2 is a schematic view of the clock;

FIG. 3 is a diagram of the signal obtained with the clock;

finally, FIG. 4 illustrates the result obtained, according to an energylevel diagram to compare with that of FIG. 1.

The core of the clock (FIG. 2) is a cell filled with an alkali gas. Anexciter 2 transmits energy to this gas in the form of a flux ofpolarised photons passing through a circular polariser 3. The excitermay also be a field of microwaves for example. It will then be necessaryin any case to inject a light beam (for example a laser) to detect theresonances of the gas. A photodetector 4 collects the luminous energyreturned by the gas of the cell 1 and transmits a signal to a countingdevice 5. A frequency separator 6 collects the signal at the output ofthe counting device 5 and transmits its results in the form of anindicator of intensity of the spectral lines measured to a device foroperating 7 the clock and a control device 8, which governs the exciter2 as well as means for applying magnetic fields 9 and 10. The latteremit magnetic fields at radiofrequencies of pulsations noted Ω and ω,which are mutually perpendicular and of direction depending on thepolarisation (for example perpendicular to the light rays emitted by theexciter 2 in the case of a circular polarisation).

Reference is made to FIG. 3. The signal coming from the counting device5 comprises several light rays, and firstly one that is at the usefulfrequency f₀ corresponding to the return of the photons by the gaseousmedium and which gives the reference to the time measurement. It againshows spectral lines at the frequencies Ω/2π, (ω−Ω)/2π, ω/2π, and(ω+Ω)/2π. These spectral lines appear for magnetic fields of low values,very much less than 1/δ·T_(T), where T_(R) is the relaxation time of thesub-levels and γ is their gyromagnetic ratio, characteristic of theexcited chemical element. They correspond to resonances between thesub-levels. Their amplitude is proportional to the ambient magneticfield. It is thus taught by the invention to apply a magnetic field forcompensating the essentially static ambient magnetic field, but which ismade to vary in a continuous manner in amplitude and in direction ifnecessary, so that the amplitude of these rays is reduced as much aspossible, which signifies that the compensation field has balanced outthe ambient magnetic field. FIG. 4 then shows that the sub-levels ofeach main level are at a same energy value, so that the photons returnedby the gaseous medium are all at the useful frequency f₀: thecorresponding spectral line appears in the form of a much sharper andhigher peak, the detection of which is thus facilitated.

By applying the invention, it becomes possible to omit traditionalmagnetic shielding of atomic clocks.

The amplitudes of the radiofrequency fields are advantageously chosen tomaximise the amplitude of resonance spectral lines (before theapplication of the static compensation field). It is advisable torespect approximately the equalities γHω/ω=1 and γHΩ/Ω=1, where Hω andHΩ are the amplitudes of the radiofrequency fields of pulsations ω andΩ. Advantageously, the means for applying the substantially staticcompensation magnetic field are identical to those that applyradiofrequency magnetic fields.

The unique exciter may be a flux of photons such as a laser flux emittedfor example by a diode laser or a lamp. The gaseous element may consistof ⁸⁷Rb, ¹³³Cs, with mixing if necessary with a buffer gas. The materialof the cell 1 may consist of a glass such as Pyrex (registeredtrademark). The means for applying magnetic fields 9 and 10 may consistof triaxial coils, or of three mutually concentric monoaxial coils. Thephotodetector 4 may be of any type measuring a flux of photons at theoutput of the cell 1. These photons have to be polarised for example bypolarisers added to the exciter. The control is accomplished by anyknown material comprising a computing unit. The coils are current orvoltage controlled. The excitation to the resonance frequency f₀ isaccomplished by an amplitude modulation of the diode laser at thefrequency f₀/2 or by a microwave cavity resonating at the frequency f₀.An exciter comprising two lasers, the difference in frequency of whichis f₀, may also be envisaged.

Since all the sub-levels become equivalent in zero field (independentlyof their m value), it is then possible to use other gases than thosenormally used (alkaline gases) in atomic clocks, in particular gases inwhich the hyperfine structure of their atoms does not have sub-levelswith zero angular momentum, such as ³He.

In a concrete example where the clock operated with ⁸⁷Rb, and awavelength of the returned photons of 795 nm, the radiofrequency fieldshad frequencies Ω/2π and ω/2π of 10 kHz and 45 kHz, and respectiveamplitudes of 15 mGauss and 70 mGauss. With an ambient magnetic field ofthe order of 10 mGauss, the compensation was made with residual fieldless than 10 mGauss at each axis. The resolution on the compensationmagnetic field (magnetic noise due to disruptions of the control means)was of the order of 0.1μ Gauss/√{square root over (Hz)}. The frequencystability of the clock was of the order of 0.67 Hz/√{square root over(Hz)}, i.e. 10⁻¹⁰/√{square root over (τ)} in relative resolution on thefrequency delivered by the clock for an integration time τ.

The use of the invention has been described for an atomic clock, whichmay be of sequential or Ramsay fringe operation. It could also beapplied to the measurement of magnetic fields, in other words asmagnetometer. It would suffice to record the compensation magnetic fieldapplied when the spectral lines of FIG. 5 due to the radiofrequencyfields would be at the minimum, reading it on the means for applyingmagnetic fields 8 and 10: the ambient magnetic field would be opposed.

It is interesting to note that since the field seen by the atoms iszero, a magnetic shield proves to be superfluous.

1-8. (canceled)
 9. An apparatus comprising a cell filled with a gas, anexciter of the gas to make its atoms jump to a higher energy level, adetector to collect a light signal passing through the gas, comprisingmeans for applying magnetic fields, applying an essentially staticmagnetic field and two oscillating magnetic fields and directedperpendicular to each other, and means for controlling the means forapplying magnetic fields to regulate in direction and in intensity theessentially static magnetic field, which is a magnetic field forcompensating an ambient magnetic field, so as to reduce the amplitude ofthe spectral lines of the light signal passing through the gas among thefrequencies (Ω/2π, ω/2π) of the oscillating magnetic fields, the sum((ω+Ω)/2π) of said frequencies and the difference ((ω−Ω)/2π) of saidfrequencies.
 10. The apparatus according to claim 9, wherein saidapparatus is an atomic clock.
 11. The apparatus according to claim 9,wherein said apparatus is a magnetometer.
 12. The apparatus according toclaim 9, wherein the means for applying magnetic fields comprise atleast one triaxial magnetic coil.
 13. The apparatus according to claim9, wherein the means for applying magnetic fields comprise at leastthree concentric monoaxial coils.
 14. The apparatus according to claim9, wherein the gas is chosen among alkaline gases and helium
 3. 15. Theapparatus according to claim 9, wherein said apparatus is devoid ofmagnetic shielding.
 16. The apparatus according to claim 9, whereinoscillating fields have intensities (Hω, HΩ) equal to respectivepulsations (ω,Ω) of said oscillating fields, divided by a gyromagneticratio of the gas.